AC solid state power controls are often used in various applications to control operation of an AC input power source. During operation, the power control controls power to a load. The power control itself may be an assembly containing multiple power modules and an output stage. As is known in the art, an AC power source generates an output voltage in the form of a sine wave whose voltage equals zero two times in every period. The point at which the sine wave crosses zero voltage is called a “zero crossing.”
The power control may contain an instantaneous current trip to protect the power control from potentially damaging high inrush or short circuit currents. The instantaneous current trip protects the power control output stage when turning on during short circuit conditions.
When the output stage receives instructions to switch to an ON state, the output stage turns on at the zero voltage crossing of the input power source. Because there is no current at the zero crossing, the current through the power control increases gradually as the sine wave progresses. If the output stage were turned on at a non-zero crossing in the middle of the sine wave, the instantaneous slope of the sine wave (i.e., dv/dt) would be very high, causing high inrush currents to travel through the power control undesirably. By contrast, when the device turns on at zero crossing, the dv/dt of the device output is negligible, keeping the current increase in the power control gradual.
In practice, however, external events may cause the source voltage to be applied on the output stage at non-zero crossing. For example, if a contactor in the power source is opened momentarily, or if the power source is first switched on or switched to a different power source and the solid state power control is already on, reapplying power to the power control (e.g., by reclosing the contact or connecting the power control to another power source) or applying power in the first instance, the power may not be applied precisely at the zero crossing. This may cause a high inrush current to occur, due to the higher dv/dt, than a zero crossing. This application of power at the non-zero crossing may cause a much greater inrush current, one that may exceed the instantaneous current protection of the solid state devices. The device would then switch off to protect itself. This switching of the input power source causes an abnormal condition that may lead to the solid-state device properly protecting itself, but leads to operation delays and inconvenience. Normal operation of most electric power systems will result in these momentary power input interruptions.
There is a desire for a way to detect these input voltage interruption conditions and provide a more reliably solid state power management to avoid tripping.